The field of the invention is permittivity measurement methods and devices.
The permittivity of a specimen is the complex ratio of the capacitance between a pair of electrodes which sandwich the specimen and that of the same pair with an air gap. Measurement of capacitance and determination of permittivity is generally accomplished through the use of a test fixture coupled to a measurement instrument such as an LCR meter, an impedance/material analyzer, or a network analyzer.
Test fixtures may be classified according to the measurement technique which they employ, and thus may be classified as being a parallel plate fixture, coaxial probe, transmission line fixture, free-space fixture, or a resonant cavity fixture. Parallel plate fixtures provide two parallel plates which are essentially electrodes between which a specimen is placed for measurement. Although such fixtures have many desirable characteristics, they are not suitable for measurement using signal frequencies greater than 1.8 GHz. Although other types of fixtures are suitable for use for frequencies greater than 1.8 GHz, their use is often undesirable for other reasons.
As an example, coaxial probe devices essentially transmit a signal into a specimen and examine any reflected portion of the signal picked up by the probe. Unfortunately, because of the need for the material to reflect back a significant portion of the signal, coaxial probes are generally not suitable for thin specimens (i.e. less than or equal to 1 cm). Transmission line fixtures are also problematic because they require that a specimen be shaped to fit within a transmission line such as a wave guide or a coaxial transmission line so that the effects of the specimen on a signal transmitted through the line can be examined. Free space systems broadcast a signal at a specimen through free space and examine the effect of the specimen on the signal. The use of such systems generally require that the specimen be large, flat, thin, and parallel faced, and requires tight control of the distance between an antenna to a sample. Resonant cavity fixtures are similar to transmission line fixtures in that a precisely shaped specimen is placed within a resonant cavity or a transmission line and the effects of the specimen on fields within the cavity/line are examined. As with transmission line fixtures, having to precisely shape a specimen is generally not a desirable aspect of use of the fixture.
Due to the inadequacies described for non-parallel plate fixtures, it is desirable that new parallel plate fixtures which permit the use of frequencies greater than 1.8 GHz be developed.
The present invention is directed to methods and apparatus which facilitate the measurement of the permittivity of thin films using a parallel plate device for frequencies greater than 1.8 GHz. More specifically, the use of a short cylindrical metal cavity enclosing two parallel plates/surfaces is used as a fixture for permittivity measurement of thin film. The use of such a cavity is particularly advantageous when swept frequency measurement methods utilizing frequency ranges from 0 to 20 GHz are employed.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.